(a) Field of the Invention
The present invention relates to a sheet-fed press used for multicolor printing and, more particularly, to a press plate registering method wherein register information is obtained by using the edges of press plates as references.
(b) Description of the Prior Art
In a sheet-fed press for multicolor printing, press plates are respectively wound around as many plate cylinders as the number of colors used in the printing process, and printing is sequentially carried out for each color. In this case, high printing quality cannot be obtained unless each color printing is made sequentially at the correct position on a sheet of paper. Accordingly, a registering operation is conducted for each of the plurality of plate cylinders.
Register errors may be classified as parallel misregister where color plates are out of register in parallel to the axial and/or circumferential direction of the plate cylinder, and distortion errors which occur when an undesired shift in the axial direction of the plate cylinder is combined with misregister in the circumferential direction of the plate cylinder.
If registering is effected while printing is carried out with press plates actually attached to the respective plate cylinders, a large number of sheets of paper are wasted. Therefore, such a registering method is uneconomical, and it also involves a great loss of time and requires skill in the registering operation.
Japanese Patent Laid-Open No. 229268/1992 discloses a technique in which a press plate is registered on the basis of cross-shaped marks (register marks), which are reference marks put on the press plate, and the press plate is attached to a plate cylinder by using a U-shaped hole made by punching.
However, the disclosed technique provides no register information about alignment between a plurality of press plates used for multicolor printing, although it is convenient for attaching a press plate to a plate cylinder. Accordingly, it is still necessary to carry out a registering operation after printing separately. In addition, the measuring device itself needs a punch, a centering mechanism and a press plate moving mechanism and hence becomes complicated and costly.